venting help lennox slp98 70,000 propane

venting intake/exhaust for lennox slp98v 70k has 2" fittings on furnace
I'll be running hopefully just a 45 and a 90 about 6 ft then 16 ft out.
manual specs for 2 or 2 1/2 or 3inch. Any advantage to sizing one vs the
other, ie capacity/flow 3" (bigger better) or 2" better as smaller btu not
venting great distance?

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Typically you would want to pick the vent sizing that is as small as possible that will fit into the venting table in the installation manual. The bigger the venting pipe the more expensive it is. It won't make any difference on performance and will sometimes hurt the longevity of the venting motor if the vent sizing is too large. Be sure to follow the instructions as to grade of venting pipe and support distances between hangers etc.

I beleive you should be able to use the 2" without an issue on a 70K model and only 25 feet of pipe, but I am not familiar with that exact model #.

2" is best. You can go 86 feet with 2". If you live at an altitude over 7500 feet then you can go 61 feet with 2". That furnace has a huge tolerance for venting. Oversizing would not be good as it may have a hard time calibrating the pressure switches. We don't install drip tees in the flue of that unit. York and Rheem seem to want/like that but not required with Lennox. Are you a Lennox dealer or where did you get it from. Lennox usually does not sell those unless you are trained with them. The bugs are minor and have been corrected (new burner cover) and another technical issue.

2" is best. You can go 86 feet with 2". If you live at an altitude over 7500 feet then you can go 61 feet with 2". That furnace has a huge tolerance for venting. Oversizing would not be good as it may have a hard time calibrating the pressure switches. We don't install drip tees in the flue of that unit. York and Rheem seem to want/like that but not required with Lennox. Are you a Lennox dealer or where did you get it from. Lennox usually does not sell those unless you are trained with them. The bugs are minor and have been corrected (new burner cover) and another technical issue.

You haven't had the problem of excess condensate locking them out yet?

We have sold about a hundred of them since last fall. Most install manuals of all brands recommend the furnace be sloped forward. That unit drains great to the right side but if vented to the left needs a bit of slope. Has to do with the speed/rate that the condensate drains out of the secondary coil on high fire. Have not had any problems now that our installers know about it. Had to go shim up about a dozen of the others we installed.